The primary function of a valve is to ensure that a fluid flows only in one direction. There are many valves in the human body, helping to make sure that homeostasis is maintained in an efficient manner. There are four valves in the heart that regulate the direction and flow of blood. The pulmonary valve directs blood from the right ventricle into the main pulmonary artery where it flows into the lungs. The aortic valve directs blood towards the aorta, the main artery of the body. The mitral and tricuspid valves are internal to the heart, directing blood flow between the atria and ventricles. Valves may fail to function due to inadequate opening (also known as narrowing, or stenosis), inadequate closure (also known as leaking, or regurgitation or insufficiency), or a combination of both processes. The aortic valve is the one that most commonly fails in older adults. In children, the pulmonary and aortic valves are the ones that most often need to be repaired or replaced due to congenital defects.
Pulmonary insufficiency (regurgitation) can be a result of congenital heart disease. Typically this condition would be exhibited by patients who have had previous repair of Tetralogy of Fallot. Another indication would be insufficiency of a right ventricle-to-pulmonary artery conduit, also seen in Tetralogy of Fallot or pulmonary atresia with ventricular septal defect (VSD), and also after previous repair of truncus arteriosus.
The traditional way to replace cardiac valves in patients involves open-heart surgery. The patient must be placed under general anesthesia for several hours. The patient's sternum is cut open, and incisions are made through the various tissue layers and into the heart to expose the area of interest. A replacement valve is implanted. A cardiac bypass machine must be used to pump blood while the heart is stopped. This type of surgery causes significant trauma to the patient with prolonged healing times.
The replacement heart valves currently available for traditional open-heart surgery have both advantages and disadvantages. Mechanical valves are long lasting and durable, but may require a lifetime of anticoagulation (anti-clotting) treatment. The mechanism of closure may damage blood cells and platelets, as well, it may be audible to the patient. Biological valves, usually harvested from pigs or cows (eg. Bovine jugular veins), do not generally last as long as mechanical valves, as they tend to calcify and stiffen. Their advantage is that they do not require that patients take anticoagulation treatment. As well, with growing concerns over the possible spread of disease from animal tissues (eg. Mad cow disease), it would be ideal to avoid the biological tissues altogether. Children with congenital heart disease may require several operations over a period of years to correct multiple cardiac defects. Repeated surgical repair or replacement of cardiac valves places an additional burden on them.
A biological valve for surgical implantation is sometimes mounted into a support structure called a stent. This stent is a supportive material that is surgically sutured into the valve. A biological valve supported by a stent that is inserted surgically can be referred to as a stented valve. Biological valves can be stented valves or unstented valves.
The rapidly developing specializations of interventional cardiology and interventional radiology involve placing tools and devices deep into the body under imaging guidance through small incisions percutaneously(“through the skin”) into veins or arteries. These procedures are less invasive than traditional open-heart surgery, allowing the patient to recover more quickly. The procedures take less time to perform, are safer, and are generally less costly. Percutaneous technologies therefore benefit both the patients and the health care system.
Due to the benefits of percutaneous procedures over open surgery, there has been increasing interest in developing percutaneous methods of replacing heart valves. Prior art systems include valves that can be delivered and deployed percutaneously. Progress has been made with the designs, however none of the designs are ideal, each having important limitations.
Bonhoeffer is a researcher that has performed clinical trials on animals and humans. A pulmonary valve has been percutaneously implanted in several patients. The valve is composed of an 18 mm bovine internal jugular vein with a native valve that has been dissected and reduced in profile. The valve is mounted on an expandable stent, and loaded into a specially made 18 F sheath. The system is delivered over a guide-wire and deployed using an expandable balloon. This research has been documented in several scientific papers. The valve and stent are disclosed in European Patent Application No. 1057460.
There are many disadvantages to the Bonhoeffer valve. The main one is that a large sheath is required (18-22 F) for implantation of the valve, too large to be used in small children. Bonhoeffer employs biological material as his choice for the valve. Biological materials are known to have less durability. Biological valves experiences calcification after a relatively short time period and cease to function adequately. In addition, obtaining biological material and forming it into a valve is difficult and time consuming. The biological material must be treated chemically, in effort to improve longevity and to reduce the risk of disease transmission.
Cribier is another researcher, focusing on the percutaneous implantation of aortic valves in adults. The Cribier valve that has been used in patient trials was made of either bovine pericardium or equine pericardium. The stent was 14 mm in length, and was delivered on a balloon 30 mm in length with a maximal diameter of 23 mm. The delivery profile for the assembly is even larger than that of Bonhoeffer's at 24 F.
U.S. patent application Ser. No. 2003/0023300, World Patent Application No. 02/47575, and U.S. Pat. Nos. 6,440,164 and 6,503,272 all disclose a percutaneously delivered non-biological valve mounted on a stent, which can also be referred to as a valve-stent. A main disadvantage of these designs is that the valve leaflets are supported by metal that is either an extension of the stent, or welded to it. The metal is expected to move into different positions, thereby opening and closing the orifice with each heartbeat. This will place considerable stress on the joint, potentially causing early failure of the valve due to metal fatigue.
Biological valves harvested from animals have benefits, and are therefore selected for many designs in the prior art. However, there are also many disadvantages to their use. Biological valves calcify and stiffen, which leads to deterioration in valve function over time. They require a large sheath size for introduction into the body, increasing the risk of vessel trauma in adults, and excluding their use in children. Finally, biological valves are difficult to work with. Each valve involves the slaughter of an animal, then a manual harvesting of the valve, careful chemical treatment, and hand assembly of the valve. The process is cumbersome, and labor intensive. U.S. Pat. Nos. 6,582,462, 6,425,916, 5,957,949 and World Patent Application No. 0047136A1 all disclose a biological valve as a preferred embodiment and suffer from the described disadvantages.
A healthy heart of an average human beats about 70 times a minute. This translates into approximately 600 million beats over a 15 year period, which is the FDA requirement for a mechanical heart valve. The FDA requirement for a biological heart valve is that it must last 5 years, or 200 million beats. Therefore, a heart valve must be designed and engineered to be rugged and durable. A general engineering principle is that simpler designs last longer, the smaller the number of components and parts, the lower the probability of component failure. Therefore, in designing a heart valve, the number of different materials, sutures and bonds should be minimized. United States Patent Application No. 20020198594A1 and World Patent Application No. 03047468A1 involve complex designs, with a large number of sutures and bonds, as well as different materials for the leaflets and stent graft.
U.S. Pat. No. 6,287,334 discloses a valve made out of synthetic bioprosthetic material. Three cone shaped cavities open and close with regards to the pressure differential of the fluid, thereby creating an opening in one direction, and closing in the other. This design features three independent cones, increasing the likelihood of valve dysfunction if one of the cones does not open in response to a change in pressure. This may occur if the material sticks to itself because of the adhesive attraction property of water. A second drawback to this design is that a cone is not a natural shape for a valve. Blood may pool at the bottom of the cone, stagnate, and form clots.
United States Patent Application No. 20030130729 seeks to solve the above problems by using only a biological material, and folding it in such a way as to create leaflets. The rectangular piece of biological material needs to be treated in a certain manner, dehydrated, folded and re-hydrated to achieve the desired result. The end result of the folding should be leaflets inside of a tube that is bonded or sutured to the stent. A problem with this design is that a suture line is created, where the two edges are connected to form a tube. This suture line interferes with the operation of the leaflets. It also acts as an opening for blood to seep into the space between the graft and artery, increasing the risk of clot formation.
U.S. Pat. No. 5,855,601 discusses a valve mounted on a stent, in which the leaflets and graft are formed from a single piece of material. Another embodiment is for the cusp to be secured to the outer side of the stent. If the first embodiment is selected, the leaflets may stick to the graft if both are made out of a synthetic plastic-like material. If the second embodiment is selected, then the leaflets on the inside will rub against the metal of the stent, thereby wearing out faster and reducing the life of the valve. In both embodiments, no mechanism is described for preventing the leaflets from inverting instead of closing with an alternating pressure differential. An inversion of leaflets is a hazardous situation, rendering the valve non-functional, and causing severe insufficiency.
United States Patent Application No. 20030109924 also seeks to solve the above problems, by using a continuous material to form a valvular structure that can deform to block the flow of fluid. The valvular structure collapses the same way each time along stiffened zones. The shape of the material is a truncated hyperboloid shape, the base is large, and ends in a smaller neck. A disadvantage of this design is that the orifice area is reduced. In fact, this patent acknowledges the smaller orifice area, and admits that this invention can only be used in elderly patients, as their cardiac output is lower.